This invention relates to implantable medical devices and more particularly to implantable cardiac stimulating devices and monitors. More particularly, this invention relates to sensor circuitry for such cardiac devices. The sensor circuitry can be configured to act as either a cardiac wall motion sensor to provide signals indicative of cardiac mechanical activity, or as a body sensor to measure overall physical activity.
Cardiac Wall Motion Sensors
One particular use of cardiac wall motion sensors is in the context of regulating cardiac stimulation. Implantable cardiac stimulating devices may be used to provide therapy in response to a variety of pathological cardiac arrhythmias. In particular, implantable cardiac stimulating devices may be capable of providing "tiered therapy," in which the type of electrical stimulation provided by the device is determined in accordance with the severity of the arrhythmia, with more aggressive therapies being applied in response to more severe arrhythmias. Thus, a cardiac stimulating device may respond to a relatively mild occurrence of tachycardia by delivering antitachycardia pacing pulses of approximately twenty-five to thirty microjoules in a sequence known to interrupt such an arrhythmia. In response to a relatively more severe occurrence of tachycardia, the device may deliver a low energy shock on the order of approximately two to five joules, either in combination with, or as an alternative to, antitachycardia pacing pulses. In response to an occurrence of an even more severe arrhythmia (ventricular fibrillation for example), the implantable cardiac stimulating device may deliver a high energy "defibrillation" shock on the order of approximately ten to forty joules.
Implantable cardiac stimulating devices are also employed to provide pacing pulses to cardiac tissue for the purpose of maintaining a heart rate at a physiologically acceptable rate (i.e., to provide "bradycardia pacing support"). Bradycardia pacing support may be provided by a dedicated pacemaker, or by a device that is also capable of providing other forms of therapy, such as tiered therapy.
Effective delivery of therapy from an implantable cardiac stimulating device depends upon accurate measurement of intrinsic cardiac activity. In the case of an implantable cardiac stimulating device capable of providing tiered therapy, the device must not only be capable of detecting the onset of an arrhythmia, but must also be capable of discriminating among various types of arrhythmias in order to deliver an appropriate form of electrical stimulation therapy. For example, if ventricular fibrillation is incorrectly diagnosed by the device as a relatively less severe arrhythmia, valuable time may be lost if an inappropriate, less aggressive therapy, such as antitachycardia pacing, is applied. Conversely, if tachycardia is incorrectly diagnosed as ventricular fibrillation, the patient may consciously experience high energy defibrillation shocks, which, although effective in terminating the tachycardia, are extremely uncomfortable, and may cause unnecessary myocardial stunning due to the defibrillation shock.
Measurement of intrinsic cardiac activity is also desirable for implantable cardiac stimulating devices (so-called "demand pacemakers") capable of providing bradycardia pacing support. Typically, the delivery of bradycardia pacing pulses from such devices is inhibited in response to spontaneous cardiac depolarizations (R-waves) which occur within a predetermined time period (commonly referred to as the "escape interval") following a preceding depolarization (R-wave). For example, if the intrinsic heart rate of a patient during a particular time interval is greater than a programmed rate, delivery of pacing pulses may be inhibited during that time interval. Pacing pulses would be provided when the intrinsic heart rate falls below the programmed rate. Pacing pulse inhibition is safer because it avoids competitive pacing, and is desirable because it extends battery life by avoiding delivery of unnecessary stimulation pulses. In order for a device to be capable of inhibiting delivery of pacing pulses, it must be capable of detecting intrinsic cardiac activity.
Many implantable cardiac stimulating devices that detect and discriminate among cardiac arrhythmias monitor heart rate, which is usually accomplished by measuring cardiac electrical activity--i.e., the intracardiac electrogram (IEGM). The IEGM is typically sensed by electrodes that are also used to deliver electrical stimulation therapy to the cardiac tissue. However, under some circumstances, it is difficult to sense the IEGM. For example, the device may not be able to discern the IEGM over noise or other physiological electrical activity, or perhaps even external interference. As a result, an implantable cardiac stimulating device may have difficulty detecting the onset of an arrhythmia. As another illustration, implantable cardiac stimulating devices capable of providing bradycardia pacing support may be inhibited from sensing cardiac electrical activity during a period of time immediately following the delivery of a pacing pulse, due to the presence of a pulse-induced after-potential.
Other known implantable cardiac stimulating devices use hemodynamic signals to detect cardiac arrhythmias. For example, U.S. Pat. No. 4,774,950 of Cohen refers to a system that may detect cardiac arrhythmias by measuring mean pressure at a variety of locations (e.g., mean arterial pressure, mean right ventricle pressure, mean left atrial pressure, mean left ventricle pressure or mean central venous pressure). For a selected mean pressure, a short term current mean pressure is compared to a long term mean baseline pressure, and if they differ by a predetermined value, the patient may be deemed to be experiencing a cardiac arrhythmia. The mean pressure data may also be used in combination with heart rate measurements to detect arrhythmias.
Another example of a device that uses hemodynamics to detect cardiac arrhythmias is described in U.S. Pat. No. 4,967,748 of Cohen. In that patent, blood oxygen level is measured at a particular site in the circulatory system of a patient. A comparison is made between a short term sensed blood oxygen level and a baseline blood oxygen level, and if they differ, the patient may be deemed to be experiencing a cardiac arrhythmia.
Unfortunately, the use of hemodynamic indicators such as mean pressure and blood oxygen level may have certain associated drawbacks. One possible drawback is that hemodynamic indicators may not respond rapidly to the onset of an arrhythmia. Thus, an implantable cardiac stimulating device that relies on such hemodynamic signals to detect cardiac arrhythmias may not deliver therapy as rapidly as desired.
One proposed solution which overcomes some of the drawbacks associated with the use of the IEGM and certain other hemodynamic indicators is described in commonly-assigned, copending U.S. patent application Ser. No. 08/091,636, filed Jul. 14, 1993, now U.S. Pat. No. 5,628,777 of Moberg and Causey, entitled "Implantable Leads Incorporating Cardiac Wall Motion Sensors and Method of Fabrication and a System and Method For Detecting Cardiac Arrhythmias Using a Cardiac Wall Motion Sensor Signal," which is incorporated by reference herein in its entirety. That patent application describes cardiac wall motion sensors that provide signals indicative of cardiac mechanical activity. The sensors are incorporated into implantable leads, such as endocardial leads, myocardial active-fixation leads and epicardial patch electrodes. The sensors are accelerometer-based such that they provide signals representative of cardiac wall accelerations as experienced by the sensors in the leads.
As described in patent application Ser. No. 08/091,636 now U.S. Pat. No 5,628,777 of Moberg et al., signals from the therein-described cardiac wall motion sensor may be used to discriminate among various cardiac arrhythmias in a manner traditionally accomplished by analyzing electrocardiograms or aortic pressure signals. An implantable cardiac stimulating device may be constructed to receive both a cardiac wall motion sensor signal (which is indicative of cardiac mechanical activity) and an IEGM signal (which is indicative of cardiac electrical activity). The device may be configured to use either form of information, or both forms of information in combination, to detect and discriminate among various types of cardiac arrhythmias and to determine intrinsic heart rate. Moreover, as described in commonly-assigned, U.S. Pat. No. 5,480,412, issued Jan. 2, 1996 of Mouchawar et al., entitled "System and Method for Deriving Hemodynamic Signals from a Cardiac Wall Motion Sensor" (incorporated by reference herein in its entirety), cardiac wall displacement may be derived from the cardiac wall motion acceleration signal, and the displacement used as a hemodynamic indicator.
As described in U.S. patent application Ser. No. 08/091,636, now U.S. Pat. No. 5,628,777 of Moberg et al., the cardiac wall motion sensor may be implemented as an accelerometer constructed of a cantilever beam having one end fixably coupled to a cardiac lead that is in contact with a cardiac wall surface. A mass is disposed on the other end of the cantilever beam, which is left free to move. As the cardiac wall moves, the resulting movement of the lead causes the beam to flex. The mass can be centered, i.e., symmetrically disposed about the free end, so that the beam flexes in response to motion perpendicular to the beam. Alternatively, the mass can be offset, i.e., asymmetric with respect to the planar surface of the beam, so that the beam will flex in response not only to motions perpendicular to the beam, but also coaxial with respect to the beam. The measured acceleration of the beam provides an indication of cardiac mechanical activity.
A piezoelectric material may be disposed on both surfaces of the beam. The surfaces of the piezoelectric material are polarized to bear an electric charge. In response to upward and downward deflections of the beam, the charge on the piezoelectric surfaces change in magnitude and polarity, and thereby provide a signal indicative of the accelerations of the cardiac wall to which the lead is attached. In the offset mass configuration, the signals represent motion in both the perpendicular and coaxial directions combined.
The use of a piezoelectric material in a cardiac wall motion sensor exhibits a number of disadvantages. The piezoelectric accelerometer cannot measure a DC response (i.e., it cannot measure a constant acceleration). Further, the piezoelectric material is subject to thermal drift. As the ambient temperature of the sensor changes with time, the electrical properties of the piezoelectric material change correspondingly.
To overcome these problems, the above-referenced application also describes the use of a piezoresistive material to coat both sides of the cantilever beam. A piezoresistive material undergoes a change in resistance when the material is subjected to a mechanical stress or strain. Preferably, each side of the cantilever is coated with piezoresistive deposits to eliminate the effects of temperature drift. When compensated in this manner, the piezoresistive accelerometer exhibits a DC response and thus can be used to measure constant acceleration.
One drawback of temperature-compensated sensors results from the bridge circuitry used to cancel out the temperature effects common to both the upper and lower piezoresistive surfaces of the beam. The bridge requires three wires for each cantilever beam to measure resistance. In contrast, an uncompensated sensor requires only two conductors to measure acceleration.
The number of conductors used to measure cardiac acceleration increases when motion sensitivity in more than one direction is desired. One cantilever beam bearing an offset mass is sensitive to the vector sum of cardiac wall motion in two directions. However, such a sensor does not provide separate measurements of acceleration in individual orthogonal directions. Unfortunately, the cumulative multiaxial measurement of a single-beam sensor may register a zero or reduced value because of cancellation of nonzero measurements in different directions. Thus, it may be desirable to obtain two or more distinct measurements of the amplitude of cardiac wall motion in at least two individual noncoaxial directions.
As described above, a cardiac wall motion sensor can measure uniaxial motion by employing a centered mass, rather than an offset mass, that is symmetric about the cantilever axis. The measurement of cardiac accelerations in two separate directions would then require two cantilevered sensors. Unfortunately such a configuration implementing the temperature-compensated piezoresistive bridge design would require five wires (three for each cantilever with one wire in common), thereby occupying valuable space in the implantable cardiac lead.
Physical Activity Sensors
Accelerometer-based sensors have also been used as body sensors to measure a patient's physiological activity. For example, a rate-responsive pacemaker uses such sensor measurements to control a patient's heart rate. Accelerometer-based physical activity sensors are described in commonly-assigned, U.S. Pat. No. 5,383,473, issued Jan. 24, 1995, entitled "A Rate-Responsive Implantable Stimulation Device Having a Miniature Hybrid-Mountable Accelerometer-Based Sensor and Method of Fabrication," of Moberg; and Ser. No. 08/091,850, filed Jul.14, 1993, now U.S. Pat. No. 5,425,750, entitled "Accelerometer-Based Multi-Axis Physical Activity Sensor for a Rate-Responsive Pacemaker and Method of Fabrication," of Moberg, both of which are incorporated by reference herein in their entirety.
The utilization of accelerometer-based sensors to measure physical activity is the result of an evolution in pacemaker design. Early demand pacemakers enabled a physician to adjust the heart rate to be maintained by telemetrically adjusting the length of the escape interval. However, this technique only allowed for adjustments to a fixed "programmed rate," and did not accommodate patients who required increased or decreased heart rates to meet changing physiological requirements during periods of elevated or reduced physical activity. Unlike a person with a properly functioning heart, these patients were paced so that a constant heart rate was maintained regardless of the level of the patient's physical activity. Consequently, during periods of elevated physical activity, these patients were subject to adverse physiological consequences, including lightheadedness and episodes of fainting, because their heart rates were forced by the pacemaker to remain constant.
Later pacemakers were capable of adjusting the rate at which pacing pulses are delivered in accordance with metabolic needs of the patient. These devices, known as "rate-responsive pacemakers," typically maintain a predetermined minimum heart rate when the patient is engaged in physical activity at or below a threshold level, and gradually increase the maintained heart rate in accordance with increases in physical activity until a maximum rate is reached. Rate-responsive pacemakers typically include circuitry that correlates measured physical activity to a desirable heart rate. In many rate-responsive pacemakers, the minimum heart rate, maximum heart rate, and the slope or curve between the minimum heart rate and the maximum heart rate are telemetrically programmable to meet the needs of a particular patient.
One approach that has been considered for correlating physical activity to an appropriate heart rate involves measuring a physiological parameter that reflects the level to which the patient is engaged in physical activity. Physiological parameters that have been considered include central venous blood temperature, blood pH level, QT time interval and respiration rate. However, certain drawbacks such as slow response time, excessive emotionally-induced variations, and wide variability across individuals, render the use of certain physiological parameters difficult. Accordingly, they have not been widely applied in practice.
More generally accepted have been rate-responsive pacemakers which employ sensors that transduce mechanical forces associated with physical activity. These sensors are similar to those used to measure cardiac wall motion. U.S. Pat. No. 4,140,132 (of Dahl) and U.S. Pat. No. 4,428,378 (of Anderson et al.) describe examples of rate-responsive pacemakers that maintain a paced heart rate in accordance with physical activity as measured by a piezoelectric sensor.
U.S. patent application Ser. No. 08/091,850, now U.S. Pat. No. 5,425,750 of Moberg, described above, also provides an accelerometer-based physical activity sensor that employs a weighted cantilever arrangement similar to that disclosed in commonly-assigned copending U.S. patent application Ser. No. 08/091,636, now U.S. Pat. No. 5,628,777 of Moberg et al. The physical activity sensor is preferably mounted within an implantable stimulation device so as to be responsive to bodily accelerations associated with physical activity. The sensor employs an offset-weighted cantilever arrangement to provide sensitivity to bodily accelerations that are both perpendicular and coaxial to the cantilever beam.
In the context of measuring physical activity, the multi-axial sensitivity of such a sensor overcomes the limitations on a physician's ability to choose only the common "front-back" axis of sensitivity for a particular patient, i.e., the axis that projects from the patient's chest (as described above with respect to U.S. Pat. No. 4,144,132 (Dahl)). However, a physician may decide that other axes of sensitivity are more appropriate for a particular patient, e.g., where a patient is frequently subjected, perhaps for occupational reasons, to externally induced forces in the vertical and/or lateral directions.
Moreover, it is difficult to position a single-axis physical activity sensor so as to orient it in a predetermined direction. Unfortunately, the sensor may be subject to "twirler's syndrome"--a condition in which the patient absentmindedly manipulates the pacemaker implanted beneath the skin, thereby changing its orientation from time to time. Changes to the axis of sensitivity may lead to unexpectedly high or low measurements of physical activity which can cause a pacemaker to make inappropriate heart rate adjustments.
The multi-axial sensor described in U.S. patent application Ser. No. 08/091,850, now U.S. Pat. No. 5,425,750 of Moberg suffers the same drawbacks of the offset-weighted cantilevered cardiac wall motion sensor described above. Like the cardiac wall motion sensor, a single-beam, offset-weighted physical activity sensor provides only a measurement of the vector sum of bodily accelerations in different directions. It does not provide individual measurements along each direction of movement, although it may be desirable for a physician to know whether the measured accelerations are taking place along a particular axis. Vertical motion may indicate that the patient is climbing a set of stairs, which would place a greater strain on the heart than a front-back walking motion. Thus, such information could be useful in providing the proper cardiac stimulation. However, to obtain the same temperature compensation advantages and individual directional measurements as desired in the cardiac wall motion sensor, the physical activity sensor described above would require the same number of measurement wires as the cardiac motion sensor for each sensor configuration.
For both cardiac wall motion sensors and physical activity sensors, one concern that must always be kept in mind when designing implantable cardiac monitors and stimulating devices is the need to conserve the limited space available within the implantable device and the associated leads. Both the size and the number of components required to construct physical activity sensors and cardiac wall motion sensors should thus be kept to a minimum. Any attempt to improve directional sensitivity should avoid the use of additional hardware components, including additional transducers or wires, to the greatest extent possible.